The invention relates to systems for managing electrical energy and for controlling attitude in satellites, and a major, although non-exclusive, application thereof lies in satellites placed in low earth orbit (LEO) whose passages through the shadow cast by the Earth repeat at high frequency.
Satellites have an on-board electrical power supply network including a solar array and energy storage means enabling the payloads to be powered during periods in which the solar arrays are in shadow. They also typically include an attitude control unit that controls a plurality of reaction wheels rotating about different axes or a plurality of gyrodynes whose pivotal axes are oriented along different directions.
At present, the storage means are constituted by a battery of electricity storage cells of sufficient capacity to power the payloads during the periods in which the solar array does not receive flux or in which it receives insufficient light flux. Batteries suffer from the drawback of considerable weight. In addition, it is not possible to subject them to too many full discharges, so the amount of stored energy that is actually usable on a satellite in low orbit is relatively small.
An object of the invention is to provide a system for managing electrical energy on a satellite that satisfies practical requirements better than previously known systems; it is a more particular object to enable an increased amount of usable energy to be stored for given weight and size, without any corresponding degradation of attitude control.
To this end, the invention provides in particular a system comprising an attitude control unit controlling a plurality of reaction wheels oriented about different axes, and an on-board electricity power supply network having a primary power source such as a solar array, and storage means connectable to a power line. The storage means comprise a buffer capacitor and at least one inertia wheel with a respective motor/generator, interchanges between the storage means and the power supply line being controlled by a regulator designed to attenuate power demand transients and thus torque transients on the rotary equipment. The system can have a plurality of sets of pairs of inertia wheels or storage flywheels, oriented on different axes; in addition to one or two sets of inertia wheels, it can also have an additional wheel oriented along a third axis of a trihedron having three right angles.
Usable wheels already exist having a maximum speed of several tens of thousands of revolutions per minute (rpm).
In an advantageous embodiment, the rotary equipment comprises two contra-rotating storage wheels in axial alignment and driven at substantially the same speed, thereby reducing the total angular momentum of the rotary equipment, and thus reducing variations therein during acceleration or deceleration of the wheels. However, it is not possible to guarantee that the axes of rotation of the two wheels are in alignment with accuracy better than a fraction of a degree. The inevitable misalignment gives rise to large amounts of parasitic torque during periods of acceleration or of slowing down, particularly when power is taken or delivered suddenly. The harmful effect of this misalignment can be mitigated by increasing the attitude control capacity of the satellite. Because of physical limitations, this is not always possible, in which case it is advantageous to reduce the slope of transients in power demand from the storage wheels.
The discharge regulator can accept this power limitation on the generator(s) of the wheel(s), by calling on energy stored in the buffer capacitor, which means that the capacitor must have sufficient capacitance. Until recently, a capacitance of several Farads necessary for satisfying the needs of a satellite of the type commonly used at present for Earth observation or for telecommunications purposes would have required mass and volume that are incompatible with being installed on a satellite. However, so-called xe2x80x9csuper-capacitorxe2x80x9d components now exist that make it possible to achieve the necessary capacitances with acceptable volume and weight.
The term xe2x80x9ccapacitorxe2x80x9d as used herein should be understood as designating either a single capacitor, or else (as will usually be the case) a plurality of capacitors connected in parallel and/or series.
In practice, equipment having a single wheel would require the wheel to rotate at very high speed to make it possible to compensate for disturbing torque.
The structure defined above makes it possible not only to reduce the mass of the energy storage means, but also to expect a lifetime that is much longer than that of electrochemical storage batteries, at least when such batteries are called on frequently to stand in for solar arrays.
The above characteristics and others will appear more clearly on reading the following description of a particular embodiment, given by way of non-limiting example. The description refers to the accompanying drawings.